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PubMed1999

Myocardial oxygenation in vivo: optical spectroscopy of cytoplasmic myoglobin and mitochondrial cytochromes.

The oxygenation state of myoglobin and the redox state of cytochrome c provide information on the PO(2) in the cytosol and mitochondria, respectively. An optical "window" from approximately 540 to 585 nm was found in the pig heart in vivo that permitted the monitoring of myoglobin and cytochrome c without interference from Hb oxygenation or blood volume. Scanning reflectance spectroscopy was performed on the surgically exposed left ventricle of pigs. Difference spectra between control and a total left anterior descending coronary artery occlusion revealed maxima and minima in this spectral region consistent with myoglobin deoxygenation and cytochrome c and b reduction. Comparison of in vivo data with in vitro fractions of the heart, including Hb-free tissue whole heart and homogenates, mitochondria, myoglobin, and pig red blood cells, reveals minimal contributions of Hb in vivo. This conclusion was confirmed by expanding the blood volume of the myocardium and increasing mean Hb O(2) saturation with an intracoronary infusion of adenosine (20 microgram. kg(-1). min(-1)), which had no significant effect on the 540- to 585-nm region. These results also suggested that myoglobin O(2) saturation was not blood flow limited under these conditions in vivo. Work jump studies with phenylephrine also failed to change cytochrome c redox state or myoglobin oxygenation. Computer simulations using recent physical data are consistent with the notion that myoglobin O(2) saturation is >92% under basal conditions and does not change significantly with moderate workloads. These studies show that reflectance spectroscopy can assess myocardial oxygenation in vivo. Myoglobin O(2) saturation is very high and is not labile to moderate changes in cardiac workload in the open-chest pig model. These findings indicate that myoglobin does not contribute significantly to O(2) transport via facilitated diffusion under these conditions.

Animals↗
PubMed2002

Stereoselective transport of histidine in rat lung microvascular endothelial cells.

The transport characteristics of L- and D-histidine through the blood-lung barrier were studied in cultured rat lung microvascular endothelial cells (LMECs). L-Histidine uptake was a saturable process. The addition of metabolic inhibitors [2,4-dinitrophenol (DNP) and rotenone] reduced the uptake rate of L-histidine. Ouabain, an inhibitor of Na(+)-K(+)-ATPase, also reduced uptake of L-histidine. Moreover, the initial L-histidine uptake rate was reduced by the substitution of Na(+) with choline chloride and choline bicarbonate in the incubation buffer. The system N substrate, L-glutamic acid gamma-monohydroxamate, also inhibited uptake of L-histidine. However, system N-mediated transport was not pH sensitive. These results demonstrated that L-histidine is actively taken up by a system N transport mechanism into rat LMECs, with energy supplied by Na(+). Moreover, the Na(+)-independent system L substrate, 2-amino-2-norbornanecarboxylic acid (BCH), had an inhibitory effect on L-histidine uptake in Na(+) removal, indicating facilitated diffusion by a Na(+)-independent system L transport into the rat LMECs. These results provide evidence for there being at least two pathways for L-histidine uptake into rat LMECs, a Na(+)-dependent system N and Na(+)-independent system L process. On the other hand, the uptake of D-histidine into rat LMECs was not reduced by the addition of DNP, rotenone, or ouabain, or by Na(+) replacement. Although the uptake of D-histidine was reduced in the presence of BCH, the addition of L-glutamic acid gamma-monohydroxamate did not significantly decrease uptake of D-histidine. These results suggest that the uptake of D-histidine by rat LMECs has different characteristics compared with its isomer, L-histidine, indicating that system N transport did not involve D-histidine uptake.

Amino Acid Transport Systems↗
PubMed1987

Basolateral glucose transport by intestine of teleost, Oreochromis mossambicus.

Transport characteristics of D-glucose by isolated basolateral membrane vesicles of the teleost fish, Oreochromis mossambicus, were measured. Specific activity of the vesicle Na-K-adenosinetriphosphatase was increased 11-fold, whereas specific activities of brush-border and organelle membrane enzymes were enriched only 0.3- to 0.8-fold. Vesicles had diameters of 0.1-0.4 micron, 70% of vesicles were leaky (unsealed), and 60% of sealed vesicles were inside out. D-Glucose transport occurred by stereospecific facilitated diffusion, independent of 120 mM gradients of either NaCl or KCl, and was inhibited by sulfhydryl reagents, phloretin, and cytochalasin B, but not by phloridzin. Competition studies with a range of sugars demonstrated that aldohexoses in the C-1 chair conformation were preferred substrates and probably share the same carrier. Kinetic analysis of glucose influx yielded a Kt of 10 mM and a Jmax of 3,910 pmol X mg protein-1 X min-1. Fish intestinal basolateral D-glucose transport closely resembles that of mammalian or avian intestinal epithelia and of red blood cell plasma membrane. The magnitude of transport is much lower in fish than in other vertebrates, which may be related to lower metabolic rates in these poikilotherms.

Animals↗
PubMed1990

Urea transport across urinary bladder and salt acclimation in toad (Bufo viridis).

The fluxes of urea across the urinary bladder of the toad Bufo viridis have been studied under conditions of acclimation to tap water or 500 mosM NaCl solution. The [14C]urea fluxes were measured simultaneously with [3H]inulin to test for nonspecific leakage. The fluxes are quite high (Ktrans = 75 x 10(-7) cm/s at 5 mmol/l urea) and are similar in either the mucosal-to-serosal or the opposite direction. "Summer" rates were five to six times higher than the "winter" rates. Antidiuretic hormone (ADH), theophylline, and forskolin increased the fluxes to variable degrees (two to five times), similar in the two acclimation conditions. Phloretin inhibited the urea fluxes by nearly 50%. 1,3-Dimethylurea and thiourea, but not acetamide, competed with the urea fluxes effectively. The fluxes of urea were not affected by the osmotic water flow, although both responded to ADH. It is concluded that urea transport across the urinary bladder of B. viridis is by facilitated diffusion through a specific pathway independent of water flux. The inhibitory effect of the structural analogues on the urea flux was affected by salt acclimation, whereas most other characteristics did not differ significantly at 5 mmol/l external urea under the two conditions of acclimation.

Adaptation, Physiological↗
PubMed2001

Oxygen affinity and amino acid sequence of myoglobins from endothermic and ectothermic fish.

Myoglobin (Mb) buffers intracellular O2 and facilitates diffusion of O2 through the cell. These functions of Mb will be most effective when intracellular PO2 is near the partial pressure of oxygen at which Mb is half saturated (P50) of the molecule. We test the hypothesis that Mb oxygen affinity has evolved such that it is conserved when adjusted for body temperature among closely related animals. We measure oxygen P50s tonometrically and oxygen dissociation rate constants with stopped flow and generate amino acid sequence from cDNA of Mbs from fish with different body temperatures. P50s for the endothermic bluefin tuna, skipjack tuna, and blue marlin at 20 degrees C were 0.62 +/- 0.02, 0.59 +/- 0.01, 0.58 +/- 0.04 mmHg, respectively, and were significantly lower than those for ectothermic bonito (1.03 +/- 0.07 mmHg) and mackerel (1.39 +/- 0.03 mmHg). Because the oxygen affinity of Mb decreases with increasing temperature, the above differences in oxygen affinity between endothermic and ectothermic fish are reduced when adjusted for the in vivo muscle temperature of the animal. Oxygen dissociation rate constants at 20 degrees C for the endothermic species ranged from 34.1 to 49.3 s(-1), whereas those for mackerel and bonito were 102 and 62 s(-1), respectively. Correlated with the low oxygen affinity and fast dissociation kinetics of mackerel Mb is a substitution of alanine for proline that would likely result in a more flexible mackerel protein.

Amino Acid Sequence↗
PubMed1984

Proximal HCO3- reabsorption and the determinants of tubular and capillary PCO2 in the rat.

Studies were carried out in Munich-Wistar rats to define the CO2 partial pressure (PCO2) profile in the surface tubules and capillaries of the kidney and to relate these measurements to proximal tubular HCO3- reabsorption, renal blood flow, and O2 consumption. In euvolemic rats, PCO2 in Bowman's space (BS) was 12.5 mmHg higher than in arterial blood, indicating CO2 addition to the arterial tree as it traverses the cortex. PCO2 further rose by 3.9 mmHg between the efferent arteriole (EA) and the peritubular capillaries (PC) (P less than 0.01) and by 4.9 mmHg between BS and the early proximal tubule (EP) (P less than 0.01). In studies with paired measurements, PCO2 in EP was 1.8 mmHg higher than in the adjacent PC (P less than 0.05). HCO3- reabsorption in EP (first 0.4-1.25 mm) was 579 pmol X min-1 X mm-1 (34.3 +/- 4.6% of the filtered load). By use of a model of facilitated diffusion of CO2 across the cell, the trans-epithelial PCO2 gradient in EP can be accounted for by the CO2 generated from HCO3- reabsorption, assuming an intracellular pH of 7.3. In the vascular compartment, roughly half the rise in PCO2 between the afferent arteriole (estimated to equal BS PCO2) and PC can be accounted for by metabolic CO2 production and half by titration of blood buffers by reabsorbed HCO3-.

Acid-Base Equilibrium↗
PubMed1989

Na+-independent sugar transport by cultured renal (LLC-PK1) epithelial cells.

The LLC-PK1 cell line has been well characterized concerning its proximal tubule-like Na+-dependent active sugar transporter in the apical membrane. In this study, we investigated the uptake of the glucose analogue, 2-deoxy-D-glucose (2DOG), a paradigm substrate for the facilitated diffusion, Na+-independent sugar transporter in the renal basolateral membrane. The uptake of 0.1 mM 2-[14C]DOG by confluent LLC-PK1 cell sheets at 25 degrees C is linear at least to 10 min, at which time greater than 90% of intracellular radioactivity is 2DOG phosphate. The uptake of this analogue by LLC-PK1 cells is Na+ independent, and the transporter appears to be localized to the basolateral cell membrane. Phlorizin is a much less effective inhibitor than its aglycon, phloretin. Cytochalasin B is also an effective inhibitor, but it causes morphological changes in the cells at concentrations required to inhibit transport. Specificity studies indicate that this transport system requires a hexose with a free hydroxyl at C-1, and that the hydroxyls at C-3 and C-4 be preferably in the equatorial position. Glucose starvation causes an increased rate of 2DOG uptake. Subconfluent (cycling) cultures of LLC-PK1 cells have a threefold greater rate of 2DOG uptake than that seen in confluent (noncycling) LLC-PK1 cells.

Biological Transport↗
PubMed2001

Renal Na(+)-glucose cotransporters.

In humans, the kidneys filter approximately 180 g of D-glucose from plasma each day, and this is normally reabsorbed in the proximal tubules. Although the mechanism of reabsorption is well understood, Na(+)-glucose cotransport across the brush-border membrane and facilitated diffusion across the basolateral membrane, questions remain about the identity of the genes responsible for cotransport across the brush border. Genetic studies suggest that two different genes regulate Na(+)-glucose cotransport, and there is evidence from animal studies to suggest that the major bulk of sugar is reabsorbed in the convoluted proximal tubule by a low-affinity, high-capacity transporter and that the remainder is absorbed in the straight proximal tubule by a high-affinity, low-capacity transporter. There are at least three different candidates for these human renal Na(+)-glucose cotransporters. This review will focus on the structure-function relationships of these three transporters, SGLT1, SGLT2, and SGLT3.

Amino Acid Sequence↗
PubMed2003

O(2) affinity of cross-linked hemoglobins modifies O(2) metabolism in proximal tubules.

Previous experiments using cross-linked tetrameric hemoglobins (XLHb) to perfuse isolated rat kidneys showed that high-O2-affinity XLHb improved proximal tubule function more effectively than low-O2-affinity XLHb. To determine how function was improved, proximal tubule fragments were incubated with albumin, Hb34 [half-saturation point (P50) 34 Torr], or Hb13 (P50 13 Torr) with Po2 values ranging from 22 to 147 Torr. ATP content reflected O2 delivery to mitochondria. Both XLHb increased ATP, Hb34 with Po2 >or= 47 Torr and Hb13 with Po2 <or= 47 Torr. XLHb increased Na-K-ATPase activity (86Rb uptake) in similar Po2-dependent patterns. O2 consumption (Qo2) was measured in a closed, well-stirred chamber. Ouabain- and oligomycin-inhibited Qo2, reflecting Na-K-ATPase activity and oxidative phosphorylation, respectively, mirrored the Po2-dependent patterns of ATP and 86Rb uptake. As Po2 fell below the midpoint of XLHb desaturation, Qo2, uncoupled from oxidative phosphorylation, transiently increased. The increase was most pronounced with Hb34. Nitro-l-arginine methyl ester had no effect on Qo2. Inhibitors of NAD(P)H oxidases and diamine oxidase partially prevented the Qo2 surge with Hb34. In conclusion, facilitated diffusion accounts for Po2-dependent XLHb effects on ATP content and Na-K-ATPase and for Hb13's effectiveness in hypoxic perfused kidneys. NO scavenging was not a factor. O2-binding characteristics influence XLHb effects on mitochondria and O2-sensitive enzymes such as oxidases.

Adenosine Triphosphate↗
PubMed2001

Brain glucose transporters: relationship to local energy demand.

Glucose, the major fuel in the brain, is transported across the cell membranes by facilitated diffusion mediated by glucose transporter proteins. Essentially two types of glucose transporters are localized in the membranes of brain endothelial cells, astrocytes, and neurons. Their densities are well adjusted to changes in local energy demand.

Animals↗
PubMedDate not supplied

Fluoride deposition in the aged human pineal gland.

The purpose was to discover whether fluoride (F) accumulates in the aged human pineal gland. The aims were to determine (a) F-concentrations of the pineal gland (wet), corresponding muscle (wet) and bone (ash); (b) calcium-concentration of the pineal. Pineal, muscle and bone were dissected from 11 aged cadavers and assayed for F using the HMDS-facilitated diffusion, F-ion-specific electrode method. Pineal calcium was determined using atomic absorption spectroscopy. Pineal and muscle contained 297+/-257 and 0.5+/-0.4 mg F/kg wet weight, respectively; bone contained 2,037+/-1,095 mg F/kg ash weight. The pineal contained 16,000+/-11,070 mg Ca/kg wet weight. There was a positive correlation between pineal F and pineal Ca (r = 0.73, p<0.02) but no correlation between pineal F and bone F. By old age, the pineal gland has readily accumulated F and its F/Ca ratio is higher than bone.

Aged↗
PubMedDate not supplied

Fingernails and toenails as biomarkers of subchronic exposure to fluoride from dentifrice in 2- to 3-year-old children.

This work evaluated the use of fingernails and toenails as biomarkers of subchronic exposure to fluoride (F) from fluoride dentifrice (FD) in 2- to 3-year-old children. Ten 2- to 3-year-old children used a placebo dentifrice (without F) for 28 days, FD (1,570 ppm F as monofluorophosphate) for the following 28 days, and then placebo dentifrice for an additional 28 days, then returned to their usual dentifrices. Fingernails and toenails were clipped every 2 weeks, during the experimental period and for an additional 22 weeks. Nail F was analyzed by electrode following hexamethyldisiloxane-facilitated diffusion. There were no significant differences between fingernail and toenail F concentrations. Mean peak F concentrations occurred 16 weeks after starting the use of FD. Results suggest that fingernails and toenails may be suitable biomarkers of subchronic exposure to F from FD in small children.

Analysis of Variance↗
PubMed1994

Placental transport of nutrients to the fetus.

The exchange of nutrients between placenta and fetus involves three major mechanisms: (1) direct placental transfer of nutrients from the maternal to the fetal plasma; (2) placental metabolism and consumption of nutrients; (3) placental metabolism of nutrient substrates to alternate substrate forms. Carbohydrate is transported to the fetus as glucose which is taken up from the maternal plasma by the GLUT 1 transporter and transported to the fetus by facilitative diffusion according to concentration-dependent kinetics. Protein is transported to the fetus as amino acids by specific amino acid transporter proteins. Placental lipid transport to the fetus involves direct transporter mediated transfer of certain fatty acids as well as lipid uptake from lipoproteins, metabolic alteration in the placenta, and release into the fetal plasma. Placental size, architecture, developmental and pathological processes, and interaction with the fetus cooperate with transport and metabolic mechanisms to affect placental-fetal nutrient exchange.

Amino Acids↗
PubMed1987

Vitamins and the premature.

Vitamins are organic compounds essential in small amounts for metabolic functions. Since they cannot be synthesized in the body they must be supplied with the diet. While fat-soluble vitamins cross the placenta by single and/or facilitated diffusion the water-soluble vitamins are actively transported with the exception of vitamin C. An adequate supply of fat-soluble vitamins to the fetus seems to be dependent on maternal blood levels during gestation. On the contrary, the transfer of water-soluble vitamins is relatively independent of the levels in maternal blood. Due to their immaturity preterm and low birth weight infants require higher dosages of some vitamins compared to healthy full-term newborns.

Humans↗
PubMed2004

Enhanced cellular adenosine uptake limits adenosine receptor stimulation in patients with hyperhomocysteinemia.

OBJECTIVE: Endogenous adenosine has several cardioprotective effects. We postulate that in patients with hyperhomocysteinemia increased intracellular formation of S-adenosylhomocysteine decreases free intracellular adenosine. Subsequently, facilitated diffusion of extracellular adenosine into cells through dipyridamole-sensitive transporters is enhanced, limiting adenosine receptor stimulation. We tested this hypothesis in patients with classical homocystinuria (n=9, plasma homocysteine 93.1+/-24.7 micromol/L) and matched controls (n=8, homocysteine 9.1+/-1.0). METHODS AND RESULTS: Infusion of adenosine (0.5, 1.5, 5.0, and 15.0 microg/min/dL forearm) into the brachial artery increased forearm blood flow, as measured with venous occlusion plethysmography, to 2.9+/-0.4, 4.3+/-0.5, 5.6+/-1.1, and 9.6+/-2.1 in the patients and to 2.8+/-0.6, 4.4+/-1.0, 9.0+/-1.7, and 17.0+/-3.1 mL/min/dL in controls (P<0.05). However, adenosine-induced vasodilation in the presence of dipyridamole (100 microg/min/dL) was similar in both groups (P=0.9). Additionally, in isolated erythrocytes, adenosine uptake was accelerated by incubation with homocysteine (half-time 6.4+/-0.3 versus 8.1+/-0.5 minutes, P<0.001) associated with increased intracellular formation of S-adenosylhomocysteine (P<0.0001). CONCLUSIONS: In hyperhomocysteinemia, adenosine-induced vasodilation is impaired but is restored by dipyridamole. Accelerated cellular adenosine uptake probably accounts for these observations. These impaired actions of adenosine could well contribute to the cardiovascular complications of hyperhomocysteinemia.

Adenosine↗
PubMed2006

Delivery from episcleral exoplants.

PURPOSE: To assess the impact of an episcleral exoplant on transscleral delivery. METHODS: New Zealand White rabbits were given a periocular injection of sodium fluorescein (fluorescein, 376 Da) or an episcleral exoplant loaded with fluorescein. Two types of exoplants were tested: (1) a rigid polyethylene device, impermeable on one side and open to the sclera on the other, that contained compressed pellets of fluorescein and was sutured loosely (apposition group) or tightly to indent the sclera (indentation group) and (2) flexible refillable silicone exoplants also open to the sclera that were secured by suturing, to form a sealed episcleral chamber that was filled with a fluorescein solution. Ocular and plasma fluorophotometry were performed at several time points, and histology was performed to evaluate the effect of exoplants on the periocular tissue. RESULTS: Within 20 minutes of a periocular injection of fluorescein, peak fluorescence was visible in the anterior chamber (AC) and at later time points was displaced toward the retina; at all time points, the highest fluorescence was in the AC. For the polyethylene device indentation group, peak fluorescence was in the retina and posterior vitreous and spread to the AC over time. For the apposition exoplant group, two peaks of fluorescence were seen initially, one in the retina and posterior vitreous and one in the AC. The area under the concentration time curve (AUC +/- SE) for fluorescein concentration was 144.4 +/- 15.1 mug . h/mL for the retinal peak and 43.6 +/- 7.1 mug . h/mL for the posterior vitreous peak after injection of 5 mg of fluorescein into a silicone exoplant, compared with a retinal peak of 3.9 +/- 0.3 and a posterior vitreous peak of 0.99 +/- 0.26 mug . h/mL after periocular injection of 5 mg of fluorescein (P < 0.01 for each). Peak plasma fluorescein levels were significantly reduced in the exoplant group compared with periocular injection. CONCLUSIONS: An episcleral exoplant facilitates diffusion of fluorescein through the sclera resulting in high levels in the retina and posterior vitreous; levels are markedly increased compared with periocular injection of the same amount of fluorescein. It also reduces peak plasma levels indicating reduction of systemic absorption. This procedure provides a new approach that can be combined with sustained-release preparations to optimize delivery of agents to the retina and choroid while minimizing the potential for systemic toxicity.

Animals↗
PubMed1973

Transport of monosaccharides. I. Asymmetry in the human erythrocyte mechanism.

Transport of D-glucose across human erythrocyte membranes occurs via a facilitated diffusion process which demonstrates influx-efflux asymmetry. The mechanism of the asymmetry has been studied by estimating unidirectional fluxes in the presence or absence of trans equilibrium hexose. In the absence of transhexose, the half-saturation constant for efflux at 15 degrees C was approximately 10 mM as compared with 27 mM for influx; the corresponding values for maximal transfer rates (mumol/min per ml cell H(2)O) were approximately 51 vs. 18. The estimation of kinetic parameters, including the constant F(s), which is the ratio of maximal transfer rate/half-saturation constant, indicates a unique effect of intracellular hexose on the transfer system. Further evidence to support this conclusion was obtained by studying the effects of noncompetitive inhibitors on efflux vs. influx. N-ethylmaleimide, p-chloromercuribenzenesulfonate, and dichloroallyldiethylstilbestrol all inhibited efflux much more than influx. Glucose rendered the transport system more reactive to N-ethylmaleimide as assayed by efflux, whereas influx was much less affected. The results support the hypothesis that the transport system exists in two states. Transition from one state to the other is dependent on the presence of intracellular hexose.

Biological Transport↗
PubMed1980

Riboflavin transport in the central nervous system. Characterization and effects of drugs.

The relationship of riboflavin transport to the transport of other substances including drugs in rabbit choroid plexus, the anatomical locus of the blood-cerebrospinal fluid barrier, and brain cells were studied in vivo and in vitro. In vitro, the ability of rabbit choroid plexus to transport riboflavin from the medium (cerebrospinal fluid surface) through the choroid plexus epithelial cells into the extracellular and vascular spaces of the choroid plexus was documented using fluorescence microscopy. These studies provided further evidence that riboflavin is transported from cerebrospinal fluid to blood via the choroid plexus. The transport of [14C]riboflavin by the isolated choroid plexus was inhibited by thiol agents, ouabain, theophylline, various flavins (lumiflavin and lumichrome > sugar containing flavins), and cyclic organic acids including penicillin and fluorescein. Riboflavin inhibited [14C]penicillin transport competitively and the inhibition constant (K1) for riboflavin equaled the concentration of riboflavin at which the saturable transport system for riboflavin is 50% saturated (KT). These and other data suggest that riboflavin, penicillin, and possibly fluorescein are transported by the same transport system in choroid plexus. In vivo, the intra-ventricular injection or riboflavin and [14C]penicillin inhibited [14C]penicillin transport from cerebrospinal fluid. In vitro, various flavins (riboflavin > other sugar-containing flavins > lumiflavin > lumichrome) inhibited [14C]riboflavin accumulation by brain slices. These studies support the notions that: (a) riboflavin accumulation by choroid plexus (active transport) is quite different from that in brain cells (facilitated diffusion and intracellular trapping), and (b) therapeutically important cyclic organic acids (e.g., penicillin) are transported fom cerebrospinal fluid by the riboflavin transport system in choroid plexus.

Animals↗